Measurement in Ancient Egypt: The Royal Cubit and the First Standards of Mass

Long before the meter or the kilogram, Egypt had a working measurement system. Its central idea, keeping working measures in agreement with a master standard, is still the basis of calibration today.

Mass: seeds, stones and balances

Early trade measured mass by comparison on simple beam balances, using seeds and grains and later shaped stone weights. Metrology histories often cite the bega, a pre-dynastic Egyptian unit of mass, as one of the oldest known. The balance itself is everywhere in Egyptian art. The best-known example is the weighing of the heart against the feather of Ma’at in the Book of the Dead.

The modern carat has the same seed based origin. The word comes, through the Arabic qīrāṭ, from the Greek keration, the carob seed. The metric carat was fixed at exactly 200 mg in the early twentieth century.

For volume, the Egyptians used measures such as the hnw (often written hin), roughly half a litre.

The royal cubit

The royal cubit was about 52.4 cm long, divided into 7 palms of 4 digits each, or 28 digits. Finer subdivisions marked on surviving cubit rods reach fractions of a digit. Several of these rods are now held in museum collections.

Master cubits were made of stone. Builders used wooden or stone copies, and these were compared with the master. Later accounts describe workers checking their cubits against the master at every full moon, with severe penalties for neglecting it. Historians have not traced that detail to a primary source. But the practice it describes, regular comparison of working standards against a reference, is exactly what a calibration interval is.

The results speak for themselves. The four sides of the base of the Great Pyramid at Giza, each about 230 m long, agree to within a few centimetres.

From the cubit to today

The structure has not changed in four and a half thousand years. A master standard sits at the top, working measures are regularly compared with it, and the results are recorded. Today the master is the SI, held in Egypt by the National Institute of Standards, and the comparison is calibration with a stated uncertainty.

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